1. Field of the Invention
The present invention relates to a serial multistage flotation process for eliminating hydrophobic particles from a stream of materials to be treated, and a facility for the implementation of said process.
Such an invention may in particular be applied to deinking for recycling old paper or to the elimination of hydrophobic substances present in certain streams coming from blank pulps or the like (cotton, etc.), or in water originating from the implementation of industrial processes.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Quite particularly, old or recovered paper may be recycled into new paper using facilities making it possible to scrub this old paper.
Typically, the process implemented by these facilities first consists of mixing this old paper with water in a pulper, while adding appropriate chemical agents thereto. The paper is then disaggregated mechanically and the cellulose fibers are placed in an aqueous suspension before the various operations take place aiming to release its different contaminants. Thus, principles of separation by density deviation are applied to separate the heavy components from the light components. Likewise, filtration means are frequently used to separate the particles based on their sizes. Thus, for example, pressurized purifiers may be used with sieves with openings or holes, or canvas washers, to perform this function. Lastly, dispersed air flotation cells take advantage of the distinct hydrophobic properties of the components present to separate them.
The various aforementioned operations are typically performed one after the other along a treatment line sized to treat the overall flow rate of paper pulp.
In a more specific case of recycling inked paper (newspaper, magazine, office paper, etc.), the removal of the ink as a contaminant is a high priority: this is the “deinking” process. This ink is deposited on the surface of the paper during its printing. If this surface of the paper is only made up of fibers, the ink will be deposited thereon. Conversely, if this surface of the paper includes a layer of fillers (calcium carbonate, kaolin, etc.), the ink will be deposited on the latter and not directly on the fibers.
Although there are multiple printing processes (offset, heliography, flexography, etc.) on various paper media, they all have consequences for the deinking operations during recycling of the paper.
Whatever the case may be, during pulping of the recycled paper, the components of the paper separate by hydration and mechanical impacts. Among these components, it is possible to distinguish: cellulose fibers, whether long, medium or short, fines, essentially made up of fragments of cellulose too small to be qualified as fibers, mineral fillers and contaminants, which are for example particles of plastic, sawdust spots, inks and stickies, which are various fractionated or re-agglomerated residues, as well as chemical components present in the raw material and/or added during the process for manufacturing the treated material.
The inks do not fragment completely, and depending on the case, the operating mode changes.
The ink disaggregated into small particles can easily be eliminated by flotation or washing. Conversely, flotation is less effective on ink having remained in the form of large particles, called black points, or in micronized or even dissolved form. Document FR 2,963,627 discloses a pretreatment solution, making it possible to perform such a reduction in a parallel process, then to reunite the whole in a same flotation process.
Different techniques are currently implemented to eliminate the ink particles by flotation. In this respect, known from document EP 0,674,040 is a paper pulp deinking process in which a stream of paper pulp is circulated from top to bottom and, in the reverse direction, from bottom to top, a stream of air bubbles, knowing that the mass of paper pulp experiences successive recycling steps in the stages situated above one another in a same cell.
In short, the stream of air bubbles, injected at the bottom of the cells, crosses successively, in each stage, through the mass of paper pulp, which enters at a stage through an inlet to be removed therefrom down below at an outlet, in order to be reinjected up high at the inlet of the following stage placed immediately below it.
Pumps perform this recovery of the paper pulp at each stage, knowing that a new injection of air may also take place at this level, if necessary.
In the upper part of the cell, the foam is recovered that has formed therein and that has driven the hydrophobic particles, including the ink.
Such flotation cells have proven particularly effective and have a high performance compared with facilities including, for each flotation stage, separate treatment units installed side by side.
Document FR 2,963,627 further discloses a similar flotation process in a device that is no longer circular, but in an elongated rectangular shape, which we will call linear process as opposed to the circular process, including a parallelepiped enclosure that is subdivided into horizontal stages, using separators with openings through which the air bubbles from a lower stage can penetrate the higher stage in order to cross through the stream of materials circulating in the latter.
Specifically, at each stage, the stream of materials penetrates at an inlet orifice at a side end of the flotation unit to leave it at an outlet orifice at the opposite side end and reach, at that same side end and through an inlet orifice, the following stage, and so forth.
In order to optimize the ability to eliminate hydrophobic particles, it is necessary to perform successive aerations of the material for example using Venturi effect injectors, knowing that the air addition capacity of the injector is greatly reduced if residual air is present in the material before injection.